Apparatus and process for a polycondensation reaction

ABSTRACT

An improved process and apparatus for the production of a polyester or other condensation polymer is disclosed. In particular, polymerization is conducted in a reaction vessel equipped with a specially designed agitator that exposes the polymer melt partially filling the reaction vessel to inert gas flowing through the vessel. The agitator comprises a plurality of elements that lift a portion of a polymer melt in the reaction vessel and generate films of the polymer melt which films extends in planes that are parallel to the axis of the agitator and the flow of gas through the reaction vessel. In a preferred process, a melt of bis(2-hydroxyethyl) terephthalate, or its low molecular oligomers, obtained by esterifying terephthalic acid or transesterifying dimethyl terephthalate with ethylene glycol, is contacted with an inert gas at about atmospheric pressure in order to remove the reaction by-products and facilitate polymerization.

FIELD OF THE INVENTION

An improved process and apparatus for the production of a polyester oranother condensation polymer is disclosed. In particular, polymerizationis conducted in a reaction vessel equipped with a specially designedagitator that exposes the polymer melt within the reaction vessel toinert gas flowing through the vessel. The agitator comprises a pluralityof elements that lift a portion of the polymer melt in the reactionvessel and generate films of the polymer melt which films extend inplanes that are parallel to central axis of the agitator and the flow ofgas through the reaction vessel.

TECHNICAL BACKGROUND

Polyester production from aromatic dicarboxylic acids or their esterssuch as dimethyl terephthalate (DMT), and glycols is known. This hasbeen accomplished by stage-wise melt polymerization of the dihydroxyester of the aromatic dicarboxylic acid, or low molecular weightoligomers thereof, under successively higher vacuum conditions. In orderfor the polymerization to continue to the degree needed for mostcommercial applications, the condensation by-products, especiallyethylene glycol, must be removed from the reaction system at vacuums ashigh as 1-3 mm Hg. Such processes require costly high vacuum equipment,multistage steam jets to create the vacuum, and N₂ purged seals andflanges to minimize leakage of air into the system. Condensate from thesteam jets and organic by-products from the system end up as a wastewater stream that requires treatment and contributes to volatile organicemissions to the air. The present invention relates to a less costlypolymerization process that can be carried out at atmospheric pressure.

Atmospheric pressure processes employing an inert gas have beendisclosed in the prior art, but these suffer from one or more drawbackssuch as (1) the quantity of inert gas used is too large to beeconomical; (2) the reactor size might not be feasible forcommercial-scale operation; (3) inert-gas velocities may be too high tobe feasible for commercial-scale production, or (4) contact between theinert gas and the polymer melt in the reactor be inadequate ornon-uniform. Because of such drawbacks, the processes presently employedfor commercial production of polyester continue to be conducted underhigh vacuum. One object of the present invention is to provide furtherimprovement in a process, at about atmospheric pressure, for continuousor batchwise production of polyesters, particularly polyethyleneterephthalate, of high molecular weight. In another aspect of thepresent invention, an improved apparatus that may be employed in areaction process involving mass transfer of a volatile by-product intoan inert gas, is disclosed.

SUMMARY OF THE INVENTION

The present invention is directed to a process for manufacturingpolyesters of aromatic dicarboxylic acids and glycols in a molten statein which an inert gas is employed to assist in removing a volatilecondensation by-product, wherein the improvement comprises, employing ahorizontally disposed cylindrical reactor vessel partly filled with apolymerization reaction mass in the form of a melt, which reactor vesselis equipped with the following:

a) a reactor inlet for introducing a polymerizable feed into the reactorvessel;

b) a gas inlet for introducing an inert gas at or near one end of thereactor vessel and a gas outlet for removing the inert gas at or near anopposite end of the reactor vessel, thereby resulting in gas flow pastthe reaction mass in the reactor vessel;

c) means for maintaining the reaction mass in the molten state; and

d) an agitator that rotates on its axis during operation, said agitatorcomprising a plurality of elements that are longitudinally disposed toconvey a portion of the melt as said elements move through the reactionmass, the elements being positioned such that said elements generatefilms, the planes of the films being parallel to the central axis of theagitator and the flow of inert gas which is predominantly in the axialdirection; and

e) a reactor outlet for removing product polymer from the reactorvessel.

The agitator according to the present invention is different fromagitators used in conventional vacuum processes, which agitators consistessentially of rotating disks or screens. Such prior art agitatorsgenerate films that are perpendicular to the axis of the reactionvessel.

In a preferred embodiment of the present process, polymerization isconducted at atmospheric pressure. A dihydroxy ester of an aromaticdicarboxylic acid, or of a low molecular weight polymerizable oligomerthereof, is polymerized to a product with a higher degree ofpolymerization (DP), preferably in the presence of a polyesterpolymerization catalyst, wherein by-products of the polymerization areremoved from the system by means of an inert gas. This higher degree ofpolymerization is useful in bottles, fibers and films. This processprovides an improved method for producing linear aromatic polyesters,especially poly(ethylene terephthalate) (PET). The aromatic dicarboxylicacid used in the production of PET is terephthalic acid (TPA). Theprocess may involve the production of poly(ethylene terephthalate) fromterephthalic acid and ethylene glycol (EG) by esterification, or fromdimethyl terephthalate (DMT) and ethylene glycol by atransesterification stage, followed by polycondensation. The process isconducted at about atmospheric pressure or above, thereby avoiding highvacuum equipment and eliminating possible air contamination that causesproduct decomposition and gel formation. First terephthalic acid isesterified or dimethyl terephthalate is transesterified with ethyleneglycol to produce bis(2-hydroxyethyl) terephthalate or its low molecularoligomers, which are then contacted in melt form with an inert gas. Thevolatile reaction by-products are removed with the inert gas, so thatthe polymerization is preferably complete in less than about 5 hours,more preferably less than 3 hours, of contact time while the reactantsare kept at a suitable temperature to maintain them in the melt form soas to produce polyethylene terephthalate.

The above processes are preferably conducted in the presence of apolyester polymerization catalyst. However, a catalyst is not needed forthe esterification step if the starting material is terephthalic acid.In a preferred embodiment of the invention, a single stream of inert gasis recycled through a polymer finishing stage, a polycondensation stageand a stage wherein ethylene glycol is recovered for reuse in theprocess.

The invention is also directed to the novel apparatus described abovefor carrying out polycondensation or other reaction in which a volatileby-product is removed by mass transfer from a melt to an inert-gasstream.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 represents a schematic drawing of one embodiment of an apparatusthat is suitable for carrying out the polymerization of the invention,wherein material having a lower degree of polymerization is converted tomaterial having a high degree of polymerization.

FIG. 2 represents a schematic drawing of one embodiment of a rotatableagitator frame.

FIG. 3 illustrates a rotatable agitator frame comprising an additionalinner concentric "cage" formed by another set of agitator elements.

FIGS. 4a, b, c, and d illustrate in isometric and cross-sectional viewsof an agitator employing rectangular screens as agitator elements forthe generation of film surface.

FIGS. 5a, b, and c illustrate side and cross-sectional views of anagitator assembly consisting of concentric cylindrical wire cages.

FIG. 6 illustrates concentric octagonal wire cages that can be employedin an agitator assembly.

These figures are for the purpose of schematic illustration and are notdrawn to scale.

DETAILED DESCRIPTION OF THE INVENTION

Polymerization according to the present process can be carried out inone vessel, or more than one physically distinct vessel in series,wherein the reaction mass is polycondensed to some degree ofpolymerization in one vessel and then transferred to another vessel forfurther polymerization. The number of vessels may depend on mechanicalconsiderations related to handling of the polymeric melt as itsviscosity increases with the degree of polymerization, heat inputrequirements to volatilize the by-products of the reaction, and cost.Preferably, a single vessel may be employed to covert a prepolymer to afinal product having the desired degree of polymerization (DP).

The process of the present invention may be carried out batchwise orcontinuously. Batchwise production may be preferred for preparingspecialty polymers when the production required is not very large andstrict quality control is required particularly with respect toadditives. For large scale production for commodity applications, suchas molding resin, staple and yarn, it is more cost effective to carryout the above steps continuously wherein the reactants are fedsubstantially continuously into the processing vessels and the productsare removed substantially continuously. The rates of feed and productremoval are coordinated to maintain a substantially steady quantity ofthe reactants in the reaction vessels while the inert gas flowscountercurrently to the flow of the melt.

If two or more vessels are employed in series for conducting thepolycondensation, it is preferred that a single stream of inert gas isemployed that flows countercurrently to the flow of the melt in theprocess, i.e., the inert gas leaving a final stage of polymerization isled through the preceding stage and finally through a stage wherein theethylene glycol is recovered for reuse and the inert gas is recycledback to the final stage of polymerization.

The preparation of higher molecular weight polyesters by meltpolymerization from polymerizable monomers and/or oligomers is a wellknown process, see for instance N. P. Cheremisinoff, Ed., Handbook ofPolymer Science and Technology, Vol. 1, Marcel Dekker, Inc., New York,1989, pages 87-90; H. Mark, et al., Ed., Encyclopedia of Polymer Scienceand Technology, 2nd Ed., Vol. 12, John Wiley & Sons, New York, 1988,pages 43-46, 130-135 and 217-225, all of which are hereby included byreference. The conditions necessary for these polymerizations, which arealready generally known to the artisan, are applicable to thepolymerizations herein, modified as needed and as described herein, tomake various polyesters. These known features include items such asprocess temperatures and polymerization catalysts (if any).

As an example, polyethylene terephthalate (PET) is manufactured in thisprocess by first reacting terephthalic acid (TPA) or dimethylterephthalate (DMT) with ethylene glycol (EG). If DMT is the startingmaterial, a suitable transesterification catalyst such as zinc ormanganese acetate is used for the reaction. In a preferred process,(trans)esterified DMT/TPA is polymerized as a melt at atmosphericpressure or above by contacting the melt with a stream of inert gas (forexample, but not limited to, N₂ or CO₂) to remove the condensationby-products, mainly, ethylene glycol. Preferably, the inert gas ispreheated to about polymerization temperature or above, prior to itsintroduction into the polymerization equipment. It is preferred that theinert gas velocity through the polymerization equipment be in the rangeof 0.2 to 3 ft/sec, most preferably 0.3 to 1.5 ft/sec. The vapor leavingthe polymerization (containing the ethylene glycol byproduct) may betreated to recover the ethylene glycol for recycle to the esterificationstage or for other uses. The inert gas stream may be then cleaned up andrecycled. Thus, the overall process may operate as a closed loop systemwhich avoids environmental pollution and integrates ethylene glycolpurification and its recycle into the process.

The quantity of inert gas flow should be sufficient to carry theethylene glycol to be removed at a partial pressure of ethylene glycolbelow the equilibrium partial pressure of ethylene glycol with thereaction mass at the operating temperature. The operating temperatureduring polycondensation is maintained sufficiently high so as to keepthe reaction mass in a molten state. Preferably the temperature range isabout 270° C. to 300° C. The polymerization equipment is designed sothat the interfacial area between the melt and the inert gas is at least20 square feet, preferably at least about 30 square feet, per cubic footof the melt and that this surface area is renewed frequently. Underthese process conditions, the high degree of polymerization useful forfibers and films and other uses can usually be achieved in less than 5hours of residence time, and preferably in less than 3 hours ofresidence time.

To more reliably produce good quality product of the desired degree ofpolymerization, the polymerization should preferably be completed in areasonably short period such as less than 5 hours, preferably less thanabout 3 hours. By this is meant the average residence time of thepolymerizing mass in the process is preferably about 5 hours or less,more preferably about 3 hours or less. The polymerization is consideredcompleted when the degree of polymerization (DP) desired for aparticular application is achieved. For most common applications, suchas fibers, the DP should be at least 50, preferably at least 60, andmost preferably at least 70. By "degree of polymerization" is meant theaverage number of repeat units in the polymer, for instance forpoly(ethylene terephthalate), the average number of ethyleneterephthalate units in a polymer molecule. Exposure of the polymericmelt to high operating temperatures for prolonged period can cause chaincleavage and decomposition reactions with the result that the product isdiscolored and/or a high degree of polymerization is not achieved. Ifthe inert gas velocities are too low, polymerization takes longer. Ifthe velocity is too high it can lead to entrainment of the reaction massin the gas. In a continuous mode of operating, high inert gas velocitiesin a countercurrent direction can also hinder the flow of the meltthrough the equipment. Also, higher velocities may require largerquantities of gas flow without substantially increasing theeffectiveness of polymerization.

The quantity of inert gas flow employed to remove the ethylene glycol orother volatile byproduct that evolves is sufficiently high so that thepartial pressure of ethylene glycol or other byproduct in the gas, atany point in the process, is below, preferably well below, theequilibrium partial pressure of ethylene glycol with the melt at thispoint. Larger quantities of gas flow generally increase the rate ofpolymerization but the increase is not proportionately greater.Therefore, very large amounts of gas are not usually necessary ordesirable as large quantities may increase the size of recyclingequipment and the cost. Very large quantities may also require largersize polymerization equipment in order to keep the gas velocity in thedesired range.

In the continuous embodiment of this invention, wherein the inert gasflows countercurrently to the flow of the molten reaction mass,effective polymerization rates can be achieved with about 0.3-0.7 poundsof N₂ per pound of the melt (equivalent to about 2 to 5 moles of inertgas per mole of the polymer repeat unit) as long as the inert gasvelocity is at least about 0.2 ft/sec, preferably at least about 0.3ft/sec. The N₂ flow, however, should preferably be at least 0.2 lb./lbof polymer (equivalent to 1.5 moles of inert gas per mole of polymerrepeat unit). Larger quantities of gas flow may however be needed toobtain the preferred gas velocities.

In the process of this invention, the reactant is kept in a moltenstate, i.e., above its melting point, which for instance is about260°-265° C. for PET. At temperatures much above 300° C., decompositionreactions often cause product discoloration which interferes with thequality of the product. For PET, the reaction mass should preferably bemaintained at about 270° C. to about 300° C.

For the polycondensation to continue, ethylene glycol or other volatilebyproduct generated must be removed from the reaction mass by the inertgas. This removal is facilitated if there is a high interfacial areabetween the melt and the gas phase. To complete the polymerization in areasonably short period, the surface area should be at least about 20ft² /ft³ of the melt, preferably at least about 30 ft² /ft³ of the melt.A higher surface area is preferred to increase the rate ofpolymerization. The reaction equipment for contacting the melt and theinert gas should also be designed to frequently renew the interfacialarea and mix the polymer melt. This is particularly important as thedegree of polymerization increases and the melt becomes more viscous.

The rate of polymerization can also be increased by using a suitablepolymerization catalyst, particularly where a high interfacial area isprovided for inert gas--melt contact. The increase in the overall rate,however, is not proportional to the concentration of catalyst as theremoval of ethylene glycol starts to limit the overall polymerizationrate.

The catalyst may also increase the rates of decomposition reactions. Aneffective concentration of catalyst for a set of reaction conditions,such as temperature, gas flow, velocity and surface area, is such thatit gives the most enhancement in the rate of polymerization withoutsubstantial decomposition. The optimum concentration of catalysts ofvarious species are known in the art, or can be determined byexperimentation. It would generally be in the range of a few parts permillion parts of the polymer, such as about 5-300 parts per million.

Catalysts for facilitating the polymerization are any one or morepolyester polymerization catalysts known in the prior art to catalyzesuch polymerization processes, such as, but not limited to, compounds ofantimony, germanium and titanium. Antimony trioxide (Sb₂ O₃) is anespecially effective catalyst which may be introduced, for convenience,as a glycolate solution in ethylene glycol. Examples of such catalystsare found in U.S. Pat. No. 2,578,660, U.S. Pat. No. 2,647,885 and U.S.Pat. No. 2,789,772, which are incorporated herein by reference.

Polymers which can be produced by the present process include thosederived from one or more aromatic dicarboxylic acids and one or morealiphatic or cycloaliphatic glycols. By an aromatic dicarboxylic acid ismeant a dicarboxylic acid in which the two carboxyl groups are eachbound directly to a carbon atom of an aromatic ring. The aromaticdicarboxylic acid may otherwise be substituted with one or more othergroups which do not interfere with the polymerization, such as alkylgroups, chloro groups, alkoxy groups, etc. Examples of useful aromaticdicarboxylic acids include terephthalic acid, isophthalic acid and2,6-naphthalene dicarboxylic acid.

By an aliphatic or cycloaliphatic glycol is meant a compound containing2 to 20 carbon atoms of the formula R¹ (OH)₂ wherein R¹ is a divalentaliphatic or cycloaliphatic radical. If an aliphatic radical it maycontain one or more cycloaliphatic groups, and if a cycloaliphaticradical it may contain one or more alkyl or alkylene radicals. It ispreferred that R¹ contains 2 to 8 carbon atoms. Useful glycols includeethylene glycol, 1,3-propanediol, 1,4-butanediol, 1,6-hexanediol, and1,4-bis(hydroxymethyl)cyclohexane. Preferred glycols are ethyleneglycol, 1,4-butanediol and 1,3-propanediol, and ethylene is especiallypreferred.

Preferred combinations of aromatic dicarboxylic acids and glycols are(the polymer produced is in brackets) terephthalic acid and ethyleneglycol poly(ethylene terephthalate)!, terephthalic acid and1,3-propanediol poly(1,3-propylene terephthalate)!, terephthalic acidand 1,4-butanediol poly(1,4-butylene terephthalate)!, 2,6-naphthalenedicarboxylic acid and ethylene glycol poly(ethylene 2,6-napthoate)!, anda combination of terephthalic and isophthalic acids and ethylene glycolcopoly(ethylene isophthalate/terephthalate)!. Poly(ethyleneterephthalate) and/or poly(1,3-propylene terephthalate) are especiallypreferred products. The polymers can be made by polymerization ofvarious polymerizable ethers and/or oligomers described herein.

Dihydroxy esters of various aromatic dicarboxylic acids may be used inthe processes described herein. These are monomeric compounds that canpolymerize to a polymer. Examples of such compounds arebis(2-hydroxyethyl) terephthalate, bis(3-hydroxypropyl)terephthalate,bis(4-hydroxybutyl) terephthalate, bis(2-hydroxyethyl) napthoate,bis(2-hydroxyethyl) isophthalate, bis 2-(2-hydroxyethoxy)ethyl!terephthalate, bis 2-(2-hydroxyethoxy)ethyl! isophthalate, bis(4-hydroxymethylcyclohexyl)methyl! terephthalate, bis(4-hydroxymethylcyclohexyl)methyl! isophthalate, and a combination ofbis(4-hydroxybutyl) terephthalate and their oligomers. Mixtures of thesemonomers and oligomers may also be used to produce copolymers.

By a "polymerizable oligomer" is meant any oligomeric material which canpolymerize to a polyester. This oligomer may contain low molecularweight polyester, and varying amounts of monomer. For example, thereaction of dimethyl terephthalate or terephthalic acid with ethyleneglycol, when carried out to remove methyl ester or carboxylic groupsusually yields a mixture of bis(2-hydroxyethyl) terephthalate, lowmolecular weight polymers (oligomers) of bis(2-hydroxyethyl)terephthalate and oligomers of mono(2-hydroxyethyl) terephthalate (whichcontains carbonyl groups). This type of material is referred to hereinas "polymerizable oligomer".

The process may be used to produce various polyesters such aspoly(ethylene terephthalate), poly(propylene terephthalate),poly(1,4-butylene terephthalate), poly(ethylene napthoate),poly(ethylene isophthalate), poly(3-oxa-1,5-pentadiyl terephthalate),poly(3-oxa-1,5-pentadiyl isophthalate), poly1,4-bis(oxymethyl)cyclohexyl terephthalate! and poly1,4-bis(oxymethyl)cyclohexyl isophthalate!. Poly(ethylene terephthalate)is an especially important commercial product.

The process avoids high vacuum polymerization processes characteristicof the conventional art. Advantages of the process often are a simplerflow pattern, and/or lower operating costs and/or the avoidance of steamjets, hot wells and atmosphere emissions. The process also hasenvironmental advantages due to the elimination of volatile organicemissions and waste water discharge. Furthermore, polymerization isconducted in an inert environment. Therefore, there is often lessdecomposition and gel formation which results in better product quality.Ethylene glycol and inert gas (e.g., N₂ or CO₂) may be recycledcontinuously.

In a preferred embodiment of the process for making PET, an oligomerexiting the esterifier is prepolymerized to a degree of polymerization(DP) of about 15-30 and this prepolymer is fed to a finisher in order topolymerize it further to a higher DP of between about 50 and 150,preferably about 60 to about 120 and more preferably about 70 to about90. The finisher is maintained at a temperature greater than about 260°C. but not too high to cause polymer decomposition. A temperature rangeof about 270° C. to 300° C. is preferred. The polymerization product iscontinuously removed from the finisher. An inert gas, preferablynitrogen, is heated in a heater to a temperature of from about 280° C.to 320° C. and is introduced into the finisher to flow countercurrent tothe direction of polymer flow in order to remove volatile reactionby-products, primarily ethylene glycol. Preferably, the nitrogen isemployed in a closed loop and all processing equipment for cleaning andrecycling the nitrogen is operated at atmospheric pressure (or above, asis necessary to ensure the flow of nitrogen through the equipment in theloop). The quantity of inert gas introduced into the system issufficient so that the partial pressure of the by-products is maintainedbelow the equilibrium pressure of the by-products with the melt in orderto provide for the continuous polymerization. The quantity of inert gasmay be as small as about 0.3-0.7 pounds for each pound of PET produced.

FIG. 1 illustrates one embodiment of a reactor or finisher that issuitable for carrying out the polymerization of the invention,especially for producing high viscosity polymers having a degree ofpolymerization encountered in a finisher. The reactor comprises ahorizontal, agitated cylindrical reaction vessel 1. The reactor housing2 is conveniently constructed with a cylindrical body (shell) and endplates 4 and 6 that close off the ends of the cylindrical body. Areactor jacket 8 through which a heat transfer material is passedsurrounds the cylindrical body. An exemplary heat-transfer material isDowtherm® heat transfer fluid, commercially available from Dow Chemical(Michigan). Other methods of heating known in the art may be used, suchas hot oil heat, high pressure steam or electrical heating. A reactorinlet 14 for introducing a prepolymer feed into the reactor is shown atone end of the reactor, a reactor outlet 16 for discharging product fromthe reactor vessel is shown at the opposite end of the reactor.

The esterified DMT or TPA, or low molecular weight oligomers orprepolymers thereof, is continuously introduced as stream 3 at one endof the reaction vessel. A preheated inert gas, such as nitrogen, iscontinuously introduced as stream 7 at the other end, so as to provideflow countercurrent to the polymer flow. The nitrogen stream 9 carryingreaction by-product vapors, mostly ethylene glycol, leaves the reactionvessel as stream 11. The reaction mass flows as the polymer melt stream5. The polymerized product, polyethylene terephthalate, is removed asstream 15. The flow rates of streams 3 and 15 are coordinated to beequivalent to each other and controlled so as to provide the desiredhold up of the melt in the finisher, usually about 1 to 3 hours, whichis equivalent to a melt level at about 1/4 to 1/3 of the diameter of thevessel. The quantity of nitrogen introduced into the system issufficient so that the partial pressure of the evolving reactionby-products is maintained at less than the equilibrium pressure of theby-products in the, for example, poly(ethylene) terephthalate (PET)melt, so as to provide adequate driving force to remove ethylene glycolfrom the melt into the gas stream. The diameter of the vessel isdesigned so that the superficial velocity of the inert gas stream is inthe desired range.

In one embodiment of the process, use of Dowtherm® heat transfer fluidor other heating means is eliminated by employing the preheated nitrogenstream itself for heating. In this embodiment the nitrogen stream isfirst led through the heating jacket 8 in FIG. 1 to maintain the reactorwall above the melting point of the reaction mass, and is then fed asstream 7 to the reaction vessel.

The reaction vessel in FIG. 1 is equipped with an agitator 20 attachedvia drive shaft 18 to a drive 22 so that the agitator can be rotated ata controlled speed. The mechanical design of the agitator is such that

(a) the walls of the vessel are wiped;

(b) a large interfacial area of at least 20 ft² /ft³ of the meltpreferably greater then 30 ft² /ft³ of the melt is created;

(c) the surface area is renewed frequently; and

(d) good mixing is provided.

It is also preferred that a single agitator is used in each vessel, andthat this agitator spans about 75 percent or more of the internal lengthof the vessel.

Although shown horizontally disposed, it is possible for the reactorvessel to be positioned at a grade to facilitate the flow of reactionmelt.

The agitator can have various designs, so long as they provide thedesired surface area that is parallel to the flow of inert gas duringuse, which is predominantly axial along the longitudinal central axis ofthe reaction vessel. In one embodiment, shown in FIG. 2, the agitator 20comprises paddles 21 attached to rotatable ends 23 and 24 that rotateduring use, to form a rotatable frame.

A central axle attached to the agitator in the reactor vessel may extendoutside the housing of the reactor vessel where it is attached to amotor or drive for providing rotation of the agitator at a suitablerate.

The frame may comprise at least two sets of a plurality of arms thatradially extent from the longitudinal axis of the reaction vessel. Eachpair of arms can support a wiper, which is suitable as an elongatedpaddle. The edge of the paddle may be beveled to better wipe theinternal surface of the reaction vessel. The wipers, or wiper blades,may be set at a suitable angle to move a suitable amount of melt as theymove through the melt pool, so shedding of the melt for generating filmscan last through most of the rotation outside the pool. If the angle issuch that the space between the wiper and the cylindrical wall is toonarrow, the wipers will carry only a small amount of melt which maybecome quickly depleted by running through the clearance between thewipers and the cylindrical wall, and not enough left to generate filmsor be wiped on the inside cylindrical wall. If the angle is too large,on the other hand, more melt unnecessarily will be carried around.

The number of wiper blades and the number of arms attached to them at apoint along the length of the reaction vessel may vary. Large diametervessels would generally have more wipers. Also, there may be more wipersnear the feed end, where the melt is less viscous, and less near theproduct end where the melt is very viscous. The wipers may, for example,be 2 to 32 in number, preferably 4 to 12 in number.

The wiper-frame assembly is of mechanically strong construction towithstand the torque required to move through the viscous polymer meltand carry it. In one embodiment, cross rods are attached between thewiper blades for mechanical reinforcement.

As the wipers or paddles move out of the pool of reaction mass, theyshed the polymer melt as films that last for a short distance as thesurface tension starts to gradually pull the melt film together intothicker streams that have much less surface area. It has been observedthat the films last for about 1/2 inch when the DP is about 30-40, about1 inch at about 50 DP and about 2 inch at 60-80 DP. Therefore, tomaximize the surface area, additional longitudinal elements are placedunder the wipers, at suitable distances, over which the melt can falland continue to shed as films. It is advantageous to maintain thespacing between the elements narrow near the feed end where the melt isquite fluid and easily spreads into thin films, and to increase thespacing towards the product end where the melt is very viscous and flowsas thick films. If the spacing is too narrow, the viscous melt wouldstagnate between the elements and not generate the desired surface area.Thus, the spacing may be as small as 1/2 inch near the feed end and 2-4inch near the product end. Spacing can be optimized for a given diameterreaction vessel and speed of rotation. The longitudinal elements may berectangular bars, rods, wires, meshed screen or sheets of metal punchedout or cut to form grids of desired spacing. These may be arranged toform a "cage" or a plurality of concentric "cages" as shown in FIG. 3.

Alternatively, as shown in FIG. 4, the elements may be arranged in arectangular geometry, these rectangles being parallel to each other andextending longitudinally, again keeping the spacing larger at theviscous end and smaller at the less viscous end. The agitator may bethus built in sections that appear like a "stack" or a "sandwich" ofrectangular assemblies. These sections may be installed in the agitatorframe staggered, e.g., the plane of one section may be perpendicular tothose of the next section to as to keep the inert gas well distributedand to minimize by-passing (running through) of the melt by making thepath more torturous.

In FIG. 4, the elements are meshed screens, but these could be of otherconfigurations such as rods or punched sheets of metal. In this type ofagitator, the melt picked up by the wipers during their travel throughthe pool at the bottom, and thereafter shed by the wipers, flows alongthe rectangular elements to generate surface area.

The agitator is rotated at a rate (rpm) that maximizes the generation ofsurface area and provides frequent surface renewal. Faster surfacerenewal is advantageous for increasing the coefficient of transfer ofvolatile products from the reaction melt to inert gas but rotation thatis too fast can result in the viscous polymer melt being held as "globs"between the elements and, in fact, decrease the surface renewal rate.For attaining a reasonably good transfer coefficient it is preferredthat the surface be renewed at least once per minute. The agitator speedis also important to surface area generation. If the rotation is tooslow, sufficient melt is not lifted from the pool, or is shed too early,and all the elements do not generate films. If the rotation is too fastthe melt may be caught up as "globs" and does not flow effectively togenerate surface area. The rate of transfer of the volatile by-products,and hence the rate at which the polymer DP increases is proportional toboth the transfer coefficient (k) and the surface area (a). The rate ofrotation, or revolutions per minute (rpm) for a given agitator geometryand vessel diameter may be optimized to maximize the product k x a.Preferably, the agitator is rotated at about 1 to 60 rpm, morepreferably at about 1-30 rpm and most preferably at about 2-18 rpm.

To illustrate a "cage" type construction in detail, one embodiment of anagitator is shown in FIG. 5 in which the elements are wires, thecircumferential spacing of which varies along the length of the reactorvessel. The spacing is narrower at the feed end and wider at thedischarge end. The agitator is divided into sections, and a plurality ofconcentric "cages" can exist in each section, the number of which mayvary from section to section.

Surface area in this type of configuration is generated in two ways,first by filming of the melt circumferentially over the "cage" and,second, by drainage of the melt from the elements of one "cage" down toa smaller diameter "cage" below. The spacing and rpm are optimized so asto obtain good circumferential coverage and drainage at all points alongthe length of the agitator. The carrying of melt "globs" is minimized asdiscussed earlier. At the preferred 2-12 rpm, the spacing near the feedend may be as narrow as 1/2 inch and, near the product end, it may be2-3 inches. Thus, it is preferable to have more concentric "cages" nearthe feed end and less at the discharge end. The surface area generatedper unit length is, therefore, greater near the feed end and decreasesalong the length towards the product end as the number of "cages"decreases. To compensate for this, sections of larger spacing can bemade proportionately longer. In this manner, the surface generated ateach spacing, and hence the increase in DP at each spacing, is about thesame.

The surface area created in the reactor equals the sum of (A) the wipedsurface on the inside wall of the reactor, (B) the surface area of themelt pool, (C) the surface area of the agitator elements and those ofthe melt films generated as the agitator rotates. The area of the filmis to be multiplied by 2 to account for the surface area available formass transfer from both the sides of the films.

As the reactor size increases, contributions to the surface area from(A) and (B) decreases in relation to that from (C). Thus, for large,commercial scale finishers, most of the surface area is from the filmsgenerated by the agitator elements and the area due to (A) and (B) maybe neglected for design purposes. For example, a 7 ft. diameter×29 ft.long reactor, designed to generate 15,000 square ft. of surface area,the contribution from (A) and (B) is less than 4%.

In calculating the surface area, that could be generated with anagitator assembly being considered, it is first assumed that an optimumcombination of agitator RPM and element spacing is selected to maximizefilms generation, e.g., in the screens and wire "cage" type agitators,the screens and circumferential area of the "cages" are completelycovered with melt. The film surface area is twice the covered area toaccount for the two sides of the films. Preferably, the reactor isdesigned for a higher area to compensate for less than complete coverageduring operation under sub-optimal conditions.

The overall agitator for the reactor is conveniently built in sectionsor "spool pieces" that may be fastened together by suitable means.Fabricating the agitator in spool pieces offers the flexibility ofproviding different spacings or other variations depending on theparticular application or conditions of use.

Such sectionalized fabrication of the agitator also allows the insertionof baffles, for example discs and donuts which contribute to thedistribution of inert gas and improves contact between inert gas and thereaction mass. This also compartmentalizes the reactor longitudinally sothat when it is operated continuously it acts like a number of reactorsin series and the performance approaches that of a plug flow or a batchreactor.

The length and spacing of each Section can be conveniently determined bythe following equations in which L is the total length of the agitator,N is the number of sections desired. The length of the first section (atthe feed end) is given by the following equation: ##EQU1##

where X=number of folds increase in the DP which is equal to DP ofproduct/DP of the feed.

For subsequent sections, the nth section length is preferably defined asfollows: ##EQU2##

wherein pn is the pitch or spacing of the wires in the nth section andp1 is the spacing in the first section. The parameter pn is related top1 by the following equation.

    pn/p1=1+(X-1)(n-1)/N-1

For concentric "cages" in a given section, the spacing between theconsecutive cages equals the pitch.

The length of each section as calculated above may be rounded to aconvenient figure for fabrication, such that:

    L1+L2+L3 . . . LN=L

The wires selected for this construction are of suitable gauge and haveadequate mechanical strength to withstand the shear stresses of theviscous polymer melt. The wires may be 1/16" diameter near the feed endand of thicker gauge, for example, 3/16" diameter, near the viscousproduct end. Cross wires may be welded circumferentially at suitabledistances, e.g., 3 to 5 times the pitch or wire spacing, for mechanicalstrength.

For ease of fabrication, a long rectangular wire mat of desired pitchand cross-wires distance may be first constructed and then rolled into a"spiral", instead of constructing individual "cages," while keeping theseparation between consecutive winding of the spiral about the same asthe distance between consecutive concentric "cages," i.e., about equalto the wire spacing.

The "cages" need not be necessarily cylindrical. For ease offabrication, these may be of geometries such as hexagonal, octagonal,etc. FIG. 6 shows an octagonal assembly of wire cages as viewed from theend of the agitator. Rectangular sections of wire mats 30 are attachedto the radial arms 33 of a rotatable end. Such geometries allow the wiremats to be cut or made in rectangular sections that can be welded to theradial arms.

The reaction vessel and the agitator is constructed from a suitablematerial of construction having the adequate mechanical strength at theoperating temperature and which material, in order to produce a qualityproduct, is not easily corroded or reactive with the reaction mass so asto contaminate the product. Stainless steel is one suitable materialhaving the requisite properties.

The surface area needed to achieve a given degree of polymerization (DP)can be estimated, as a first approximation, by using the followingsimple equation which has been found to hold when polymerization isconducted under batch or plug flow conditions and a large quantity ofinert gas is employed:

    DP-DP°=k a t

In this equation:

DP=the desired product DP

DP°=DP of the feed prepolymer or oligomer

a=surface area in square feet

t=residence time or hold up time in hours

k=overall transfer coefficient for transfer of the volatile condensationby-products, mostly ethylene glycol, from the melt to the insert gas.The units are ft/hr.

The transfer coefficient, k, depends upon several factors, such astemperature, surface renewal rate, catalyst concentration and inert gasvelocity. Under the conditions of Example 1, its value was found to beabout 0.79 ft/hr.

Thus, for polymerizing a prepolymer of 20 DP to a product of 80 DP in 2hours of residence time, the surface area required, using this value fork, can be calculated as: ##EQU3##

For continuous polymerization, the reactor is preferably designed toprovide a larger surface area, such as 50-75 ft² /ft³ of melt for theabove example, to compensate for using less inert gas flow, e.g.,0.3-0.7 lb. N₂ /lb. of melt, and for deviations of the melt flow fromthe ideal plug flow. The higher than calculated surface area alsopermits operating flexibility. If the reactor has less area, the hold uptime would need to be proportionately longer than 2 hours. The agitatorconfigurations described herein can provide the required high surfaceareas.

For running the polymerization reaction continuously, it is desirablethat the residence time distribution of the melt flow be narrow, i.e.,it is closer to plug flow, and by-passing is prevented. By-passing canpotentially occur around the straight paddles and agitator elements,particularly when the melt is not highly viscous.

The reactor may also be divided longitudinally into a number ofcompartments by introducing baffles such that melt flows from onecompartment to the next and the reactor thus performs like severalsmaller reactors in series. One convenient way to achieve this is toinsert along the length of the agitator rings or donuts with an outsidediameter equal to that of the agitator. The inside diameter of thedonuts is such that the reactor operates at the desired level. Theinside diameter may be about 0.7 times the outside diameter. Disks mayalso be inserted in between the donuts to form a donut-disk-donutpattern, to keep the inert gas flow well distributed and improve contactwith melt by forcing it to go through the donut and then around thedisk, and so on. The baffles are sized such that the velocity of gasthrough, around or between them is not too high to cause entrainment orpush melt in the direction of the inert gas flow.

Similarly, another embodiment of the agitator comprises partial disks orpartial rings installed such that the inside edges are staggered at180°, i.e., alternate baffles face in opposite directions, so that inertgas will zigzag as well as swirl, creating greater turbulence and moreeffective contact with the melt, as these are rotated.

The process of this invention may also be carried out for batchwisepreparation of polyester wherein a batch of low molecular weightoligomer is charged to the polymerization equipment and contacted withthe inert gas as described until the desired high degree ofpolymerization is achieved. The oligomer is prepared by esterificationas described except that it may also be prepared batchwise either in aseparate vessel or in the polymerization vessel itself The gas and meltcontacting equipment may be similar to that described for the continuousembodiment of this invention except that it is not necessary to vary thespacing between the agitator elements along the length of the vessel.Also, compartmentalization to approach plug flow is not required. Thespacing of agitator elements should be chosen to accommodate theviscosity and flow characteristics of the final high molecular weightproduct. For batchwise preparation it is advantageous to adjust thespeed of the agitator as the viscosity of the melt increases. Initially,when the viscosity is low, the agitator may operate at as high as 100rpm but toward the completion of polymerization a low speed of about 1to 20 rpm, preferably about 2-12 rpm is desirable. Batchwise productionis suitable for economic reasons when relatively small quantities ofpolyester are to be prepared or when a strict control of additivesconcentrations is required for product quality considerations. When thequantities to be prepared are very small, it may be more economical tonot provide equipment for recycling the inert gas, or the ethyleneglycol, and discharge it to the atmosphere after rendering it harmlessto the environment by known methods such as scrubbing it thoroughly withwater and disposing off the water in an environmentally safe manner.

The invention can also be conducted in a semi-batch fashion wherein thepolymerization equipment is fed intermittently, reaction mass ispolymerized to a higher degree, and the product is dischargedintermittently.

EXAMPLE 1

This examples illustrates polymerization on a pilot scale in apolymerization reactor according to the present invention. The reactorconsisted of a nominal 6 inch diameter glass tube of 2 foot length. Itwas held inside an 8 inch diameter glass tube of similar length with thehelp of end plates so as to form an even annular space around thereactor and served as the heating jacket. The heating medium was airheated to 295°-300° C. which was introduced into the annular space atone end and flowed out from the other end.

The agitator consisted of two end pieces each with four arms in theshape of a cross. Each pair of arms held an approximately 20" long, 1"wide paddle or a wiper. Two rings were mounted inside this frame, each afew inches inside from the ends to hold four more such blades, such thatthe 8 blades formed a "cage" of slightly smaller diameter than the 6"diameter of the reactor so it could be freely rotated inside thereactor. Shafts were attached to the two cross end pieces which could berotated inside bearings provided in the center of each end plate of thereactor. The agitator was rotated by use of a motor having a variablespeed gear reducer attached to the shaft on one end of the reactor. Thetemperature of the polymer melt and the inert gas was monitored byplacing thermocouples inserted into the reactor from each of its twoends.

The reactor was charged with 9 pounds of a prepolymer of about 20 DPobtained from a commercial plant where it was made by esterifying TPAwith ethylene glycol and prepolymerizing it to a DP of about 20. Itcontained about 200 ppm antimony as catalyst. The charging was done byfeeding the solid prepolymer through a melt extruder which melted theprepolymer and heated it to about 280°-295° C. The agitator was rotatedat 12 rpm, and N2 preheated to about 295° C. was flowed through thereactor at a velocity of 0.57 ft/sec based on an empty cross-section ofthe reactor. Since the reactor was about 30% filled with melt thecontact velocity was about 0.82 ft/sec. The N₂ was introduced at one endand was discharged to the atmosphere from the other end. Thus, thereactor was essentially at atmospheric pressure. The temperature of thereaction mass was maintained at about 280° C. by controlling thetemperature of the hot air in the annulus. Polymerization was continuedunder these conditions for two hours. Samples of the polymer were takenevery half hour and analyzed for DP by gel permeation chromatography(GPC). The number average DP was found to be approximately 36, 52, 68,and 80, after 1/2, 1, 11/2 and 2 hours of polymerization, respectively.These DP values when plotted against time fit a straight line:

DP-DP°=(k a)t

with a slope=k a of 30 hr-1.

The reactor was estimated to provide on the average 4.58 ft² of filmarea which for 9 lb. of the melt translates to a value of "a" equal to38 ft² /ft³ of melt. The value of k was thus 30/38=0.79 ft/hr.

Initially, when the melt was at 20 DP, it was shed from the agitatorblades as streamlets but after a few minutes it started becoming viscousand falling as films that extended about 1/4-1/2" from the edges of theblades. As the polymerization proceeded to higher DP's the filmingbecame more pronounced. The melt extended as films 3/4-1" from theblades and towards the end the shed films extended 1-11/2". Thus, largersurface area could have been generated if additional elements had beenplaced in the agitator, under the blades, over which the melt could falland drop further as films. Also, instead of using hot air in the annularheating jacket, preheated N₂ could have been first passed through theannulus and then fed to the reactor.

EXAMPLE 2

This example illustrates a design of a prototype finisher according tothe present invention to be operated continuously at a rate of 100 to150 lb./hr. It will be continuously fed with a prepolymer of 20 DPprepared in an upstream esterifier and a prepolymerizer. The reactor isdesigned to produce product PET of about 80 DP useful for spinning intofibers or producing flakes. The reactor is 9 ft long and has a diameterof 18 inches. It has a heating jacket heated with Dowtherm® vapor. It isfitted with a 7.5 ft long agitator to leave about 9 inches of space oneach end for feed and discharge nozzles. The agitator has end pieces orplates that each extend to 8 arms which are attached to the shafts forrotation. To each pair of arms is attached a 11/2" wide wiper bladepositioned at a 45° angle to the inside wall of the reactor. Held insidethis eight wiper frame are spools of concentric "cages" of varyingpitches fabricated from stainless steel wires such that, starting fromthe feed end, there is a 9 inch long section of 1/2" pitch (and spacingbetween the consecutive concentric "cages"), then 18" length of "cages"of 1" pitch, followed by 27" length "cages" that are at 11/2" pitch andfinally 36" length concentric cages of 2" pitch where the concentric"cages" are 2" apart. Donuts and disks are inserted alternativelybetween the spool pieces so the reactor is compartmentalized to act as 4reactors in series. The agitator can be rotated at 3-12 rpm. Spools ofeach of the 1/2", 1", 11/2" and 2" pitches can provide about 57 ft² ofsurface area for a total of 228 ft² of surface area. The reactor isoperated with about 300 lb. or 4 ft³ of melt hold up. This translates toan average surface area of 57 ft² /ft³ of the melt which is about 50%more than what would be required if it performed as an ideal plug flowreactor. N₂ is flowed counter-currently to the flow of the melt at 90 to120 lb./hr. The superficial gas velocity under the operating conditionof about 1 atmosphere pressure and 285° C., based on an emptycross-section, is 0.36 to 0.48 ft/sec.

What is claimed is:
 1. A process for manufacturing a polyester of one ormore aromatic dicarboxylic acids and one or more glycols in a moltenstate in which an inert gas is employed to assist in removing a volatilecondensation by-product, wherein the improvement comprises, employing ahorizontally disposed cylindrical reactor vessel partly filled with apolymerization reaction mass in the form of a melt, which reactor vesselis equipped with the following:a) a reactor inlet for introducing apolymerizable feed into the reactor vessel; b) a gas inlet forintroducing an inert gas at or near one end of the reactor vessel and agas outlet for removing the inert gas at or near an opposite end of thereactor vessel, thereby resulting in gas flow past the reaction mass inthe reactor vessel; c) means for maintaining the reaction mass in themolten state; d) an agitator that rotates on its axis during operation,said agitator comprising a plurality of elements that are longitudinallydisposed to convey a portion of the melt as said elements move throughthe reaction mass, the elements being positioned such that said elementsgenerate films, the planes of the films being parallel to the centralaxis of the agitator and the flow of inert gas which is predominantly inthe axial direction; and e) a reactor outlet for removing productpolymer from the reactor vessel.
 2. The process of claim 1 for thecontinuous production of higher molecular weight PET, the processconducted at atmospheric pressure or above comprising contacting DHET orits low molecular weight oligomers, in melt form, with an inert gas,flowing in the process countercurrently to the flow of the reactionmelt, so that the ethylene glycol and other reaction by-products areremoved continuously and wherein the product PET is removed continuouslywith a hold up time equal to less than about 5 hours.
 3. The process ofclaim 1 wherein the process is conducted at atmospheric pressure.
 4. Theprocess of claim 1 wherein said surface area of the films is at least 30square feet per cubic foot of melt.
 5. The process of claim 1 whereinthe elements are selected from the group consisting of elongatedrectangular strips, screens, rods, wires, cut-out sheets, andpunched-out sheets.
 6. The process of claim 1 wherein the agitatoradditionally comprises wipers to wipe one or more internal walls of thereactor vessel.
 7. The process of claim 1 wherein the agitator elementsare spaced in a circular or polygonal geometry, in cross-section, at oneor more radial distances from the axis of the agitator to formconcentric cages.
 8. The process of claim 1 wherein the polyester ispoly(ethylene terephthalate).
 9. The process of claim 1 wherein theinert gas is selected from the group consisting of nitrogen, carbondioxide, and combinations thereof.
 10. The process of claim 1 whereinthe agitator is rotated at 2 to 12 rpm.
 11. The process of claim 1further comprising baffles.
 12. The process of claim 11 wherein thebaffles comprise donuts and discs, longitudinally spaced along thereactor vessel.
 13. The process of claim 1 wherein the distance betweenthe agitator elements is increased along the length from the feed end tothe product end of the reactor.
 14. The process as recited in claim 1wherein the agitator spans at least about 75 percent of the internallength of the reactor vessel.
 15. The process of claim 2 wherein theinert gas flows at a velocity of 0.2 to 3 ft/sec and an interfacialgas-liquid surface area is at least about 20 ft² /ft³ of the melt. 16.The process of claim 2 wherein the agitator elements are 1/2 to 1 inchapart near the feed end of the reactor, the spacing is increasedstepwise along the length of the agitator and the elements are 1.5 to 4inches apart near the product end of the reactor.
 17. The process ofclaim 2 wherein the polymerizable feed is an oligomer ofbis(2-hydroxyethyl) terephthalate having a degree of polymerization ofat least about 5 and polymerization to the final product degree ofpolymerization is conducted by contacting the reaction melt with theinert gas in a single reaction vessel.
 18. The process of claim 2wherein the inert gas is selected from the group consisting of N₂ andCO₂.
 19. The process of claim 2 wherein the inert gas is preheated toabout polymerization temperature or above polymerization temperatureprior to contacting it with the melt.
 20. The process of claim 1 whereinthe polyester is poly(ethylene terephthalate), poly(1,3-propyleneterephthalate), poly(1,4-butylene terephthalate), poly(ethylene2,6-napthoate), or copoly(ethylene isophthalate/terephthalate).
 21. Theprocess of claim 2 wherein the temperature of polymerization is about270° C. to about 300° C.
 22. An apparatus for conducting a condensationpolymerization in a molten state, which apparatus comprises ahorizontally disposed cylindrical reactor equipped with the following:a)a reactor inlet for introducing a polymerizable feed into the reactorvessel; b) a gas inlet for introducing an inert gas at or near one endof the reactor vessel and a gas outlet for removing the inert gas at ornear an opposite end of the reactor vessel, thereby resulting in gasflow past the reaction mass in the reactor vessel; c) means formaintaining the reaction mass in the molten state; d) an agitator thatis adapted to rotate on its axis during operation, said agitatorcomprising a plurality of elements that are longitudinally disposedadapted to convey a portion of a melt at the bottom of the reactor assaid elements move through the melt, the elements being positioned sothat said elements are adapted to generate films, the planes of thefilms being parallel to the longitudinal axis of the reactor vessel;wherein the films that are thus generated provide most of theinterfacial surface area in the reactor, and e) a reactor outlet forremoving product polymer from the reactor vessel.
 23. The apparatus ofclaim 21 wherein the agitator further comprises baffles along its lengthto improve inert gas distribution and/or reduce by-passing of thereaction mass in continuous operation.